In many power system protective relays, including microprocessor-based digital relays, the three phase AC voltages on the particular power line associated with the relay are measured and then applied, after being significantly reduced in magnitude by voltage transformers, to various protective elements in the relay, including distance elements and directional elements, among others, which use the reduced AC voltages to determine the presence (or lack thereof) of a fault on the power line. In addition, these AC power line voltages (phases A, B and C), sometimes referred to as potentials, are also used to produce the polarizing signals for various logic and other control circuits in the protective relay. Hence, the three phase AC power line voltages or potentials are critical input signals for proper operation of the relay and their accuracy is thus quite important.
When one or more of these AC voltages (A, B or C phase) are eliminated due to loss in the system of a voltage transformer, a protective fuse or similar protective device such as a molded case circuit breaker which protects all three phases, false outputs from the protective elements or other logic circuits can result. These false outputs can ultimately produce either a false trip signal which will cause a circuit breaker on the line to open, or in other cases there will be a failure to trip the line circuit breaker when there is in fact a fault on the line. Either of these results is quite undesirable.
For instance, in the case of a directional element in the relay which uses the calculated negative sequence voltage of the power line for the task of supervising distance elements and ground overcurrent elements, the correct calculation of the negative sequence voltage requires all three of the phase voltages (phases A, B and C). The advantage of using negative sequence voltage is that the resulting directional signal, specifically, the indication of a forward or reverse fault relative to the position of the protective relay, is independent of the magnitude of the load current or the direction thereof.
However, when a protective fuse for a particular phase line blows or similar local protective device operates, the resulting loss of the associated AC voltage on that phase line will result in an erroneous negative sequence voltage value, which in turn may result in the directional element giving a false fault direction.
Hence, it is important that the relay be able to determine a loss of potential, such as due to a blown fuse, declare accordingly a loss of potential condition and block use of any resulting output determination from elements or circuits such as described above.
In the case of blown fuse(s), the number of fuses that blow (one, two or three, for the three phases) at the secondary winding of the voltage transformer depends upon the nature of the short circuit or other condition on the load side (downstream side) of the fuse. In the situation where a molded case circuit breaker is used, all three potentials (phases A, B and C) are eliminated from the relay input information at once, regardless of the type of short circuit at the load. The loss-of-potential determination circuit in that particular case is designed to detect the loss of any of the phase voltage(s). The loss of a phase voltage or voltages in effect blocks the operation of those parts of the relay using phase potentials (voltages) directly or calculated results from equations which assume all three potentials to be present.
Most digital relays include loss of potential (LOP) logic which is capable of detecting whether one or two fuses are blown. In some cases, the LOP logic can also determine whether fuses are blown. Such LOP determinations are more sophisticated than merely determining whether all three potentials are missing, such as described above. A relatively recent LOP circuit which has been effective in most circumstances is shown in U.S. Pat. No. 5,262,679, which is assigned to the same assignee as the present invention.
Even with the more sophisticated LOP detection circuits, such as described in the '679 patent, however, there are particular system conditions which are not well covered. One such condition involves a power line condition of a heavy load and relatively weak voltage sources, i.e. a weak positive sequence source impedance behind the protective relay. In such a case, the necessary positive sequence voltage threshold value for the LOP circuit cannot be achieved. The LOP logic in that situation must be disabled, which is disadvantageous.
Another condition involves the completion of LOP calculations (and LOP pickup) prior to an instantaneous trip signal being produced by a protective element in the relay, when a true LOP condition exists. For instance, under certain conditions, such as where the line is heavily loaded and the magnitude of the load current is large, the distance elements in the relay will normally pickup very rapidly, creating in effect a "race" between the LOP logic which will ultimately block the output of the distance elements, and the distance elements themselves actually operating first and producing a trip of the line circuit breaker. While there are techniques which can be used to minimize this risk of a race and the instantaneous protective elements operating in extreme load conditions, even such techniques are not adequate. The only solution in such cases, which is undesirable, is to add time-delay to the instantaneous protective elements.
Accordingly, it would be desirable for an LOP circuit to be less susceptible to various system conditions and more reliable under all operating conditions.